It never rains but it pours

"It never rains but it pours" or "when it rains, it pours" means when one bad thing happens, followed by a lot of other badthings, which make a bad situation worse.

Source: http://dictionary.cambridge.org/dictionary/british/it-never-rains-but-it-pours

It's raining cats and dogs

The first idiom today is "It's raining cats and dogs" which is quite familiar to many of you. It means rain very hard. Moreover, the origin of this idiom seems rather interesting. This is a literal phrase dating from 17th century England. Back in the day, peasants used what little land they owned for crops so could not afford to keep cats and dogs on their land. As a result, people used to keep their animals on the thatched roofs of their cottages. When it rained heavily, the thatching became very perilous and slippery, causing the cats and dogs to fall off!